Complement concept of primary output with priorities order

See individual commit messages for details. They are mostly descriptive enough.

All forms of config (de-)serialization have been modified. This is a one-way migration. Config format is not versioned, and downgrading would cause kscreen to ignore newer files.

New API is meant to replace previous one, but old one is still kept for compatibility and convenience (and to let you shoot yourself in the foot when you least expect it). If you ever use `Output::setPrimary()` or `Output::setPriority()` after adding one to a config, here be dragons. You've been warned.

kscreen-doctor gets new subcommand, which will also have to be reflected in zsh completions. [The original patch](!79) got already merged after KF5 5.101 release.

XRandR backend now uses "_KDE_SCREEN_INDEX" output property, which is a 1-based uint32 priority number. Disconnected or disabled outputs should be considered as having prior...